Polycrystalline CdTe thin film semiconductor photovoltaic cell structures for use in solar electricity generation

ABSTRACT

Solar cell structures formed using molecular beam epitaxy (MBE) that can achieve improved power efficiencies in relation to prior art thin film solar cell structures are provided. A reverse p-n junction solar cell device and methods for forming the reverse p-n junction solar cell device using MBE are described. A variety of n-p junction and reverse p-n junction solar cell devices and related methods of manufacturing are provided. N-intrinsic-p junction and reverse p-intrinsic-n junction solar cell devices are also described.

This application is a Continuation Application of U.S. patentapplication Ser. No. 14/054,732, filed on Oct. 15, 2013, which is aContinuation Application of U.S. patent application Ser. No. 13/628,016,filed Sep. 26, 2012, now U.S. Pat. No. 8,580,602, issued on Nov. 12,2013, which is a Continuation Application of U.S. patent applicationSer. No. 12/505,391, filed Jul. 17, 2009, now U.S. Pat. No. 8,298,856,issued on Oct. 30, 2012, which claims the benefit of U.S. ProvisionalPatent Application Serial Nos. 61/081,625, filed Jul. 17, 2008,61/082,759, filed Jul. 22, 2008, and 61/186,788, filed Jun. 12, 2009,each of which is entirely incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to cadmium telluride (CdTe) thin filmsemiconductor solar cell structures, more particularly to highefficiency polycrystalline CdTe thin film semiconductor solar cellstructures grown by molecular beam epitaxy (MBE).

BACKGROUND OF THE INVENTION

A photovoltaic cell is able to absorb radiant light energy and convertit directly into electrical energy. Some photovoltaic (“PV”) cells areemployed as a measure of the ambient light in non-imaging applicationsor (in an array format) as imaging sensors in cameras to obtain anelectrical signal for each portion of the image. Other photovoltaiccells are used to generate electrical power. Photovoltaic cells can beused to power electrical equipment for which it has proven difficult orinconvenient to provide a source of continuous electrical energy.

An individual photovoltaic cell has a distinct spectrum of light towhich it is responsive. The particular spectrum of light to which aphotovoltaic cell is sensitive is primarily a function of the materialforming the cell. Photovoltaic cells that are sensitive to light energyemitted by the sun and are used to convert sunlight into electricalenergy can be referred to as solar cells.

Individually, any given photovoltaic cell is capable of generating onlya relatively small amount of power. Consequently, for most powergeneration applications, multiple photovoltaic cells are connectedtogether in series into a single unit, which can be referred to as anarray. When a photovoltaic cell array, such as a solar cell array,produces electricity, the electricity can be directed to variouslocations, such as, e.g., a home or business, or a power grid fordistribution.

There are PV cells available in the art, but these can be costly toproduce. In addition, PV cells available in the art might not provide ahigh conversion efficiency, from light to electricity, for a givenquantity of light. Accordingly, there is a need in the art for improvedPV cells and devices and methods for producing the same at lowerproduction costs and higher conversion efficiency.

SUMMARY OF THE INVENTION

An aspect of the invention provides a process for forming highperformance, single junction photovoltaic devices, comprising highdeposition rate polycrystalline growth using molecular beam epitaxy(“MBE”). In an embodiment, the process further provides the capabilityto do the following: in situ superstrate (or substrate) temperaturecontrol; in situ doping of the p-n junction; in situ, high doping; insitu thermal anneal; in situ grain boundary passivation by overpressureof suitable beam constituents; compositional grading during growth byflux level control of suitable beam constituents; high precision controlover layer thicknesses; and high precision control over depositiongrowth rates. In an embodiment, for the process temperature ranges fromabout 150° C. to 425° C., or from about 200° C. to 400° C., or fromabout 250° C. to 350° C. can be accommodated.

In an embodiment, doping of p-n junctions can range from 1×10¹⁶ cm⁻³ to1×10¹⁸ cm⁻³ for both p-type and n-type dopants. In another embodiment,high doping can range from 3×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³ for both p-typeand n-type dopants.

In an embodiment, for the process a thermal anneal range of about 25° C.to 125° C., or 50° C. to 100° C., above the superstrate depositiontemperature can be accommodated. Overpressures of suitable beamconstituents of about 10-30% above nominal pressure can be accommodated.In addition, flux levels of beam constituents can be varied stepwise orin a finer fashion from no flux to substantially high fluxes so as toprovide the necessary growth rates. In an embodiment, for the process,layer thicknesses can be controlled at the 10 Å level of growth orbetter.

In an embodiment, growth rates can be varied stepwise or finer fromabout 0.3 microns per hour to 3 microns per hour. In another embodiment,growth rates can be varied stepwise or finer from about 6 microns perhour to 12 microns per hour. In another embodiment, growth rates can bevaried stepwise or finer from about 18 microns per hour to 25 micronsper hour or faster.

Another aspect of the invention provides polycrystalline p-n junctionphotovoltaic cell (also “photovoltaic cell” herein) structures having atleast two layers of compound semiconductor materials, comprising ZnTe,MgTe, graded Cd_(x)Zn_((1-x))Te, and CdTe. The structure can be grown ona superstrate with or without a transparent conductive oxide (“TCO”)with successive semiconductor layers deposited to provide, in sequence,an optional thin buffer layer, a low ohmic, very high doped frontsideconnection layer, an n-p junction, and a low ohmic, high doped backsideconnection layer as the final semiconductor layer, followed by anoptional in situ metallization.

In an embodiment, a heritage molecular beam epitaxy technique, orsimilar high vacuum, free-streaming flux of elements or reactivemolecules can be operated in a mode of high deposition rate, 6-10microns/hour, to produce polycrystalline material structure with a totalthickness between about 1 micrometers (“microns”) and 4 micronsdeposited onto an optically transparent superstrate, e.g., aconductive-oxide coated piece of glass (the “superstrate”) at adeposition temperature between about 200° C. and 400° C. withsuperstrate area greater than 600 mm×600 mm.

In an embodiment of the device structure, an optional ZnTe or MgTebuffer layer of thickness less than or equal to about 200 Å can bedeposited onto the superstrate at a deposition temperature between about250° C. and 350° C. In an embodiment, a high doped layer of ZnTe ofthickness less than about 200 Å can be deposited onto the buffer layerat a deposition temperature between about 250° C. and 350° C. The highdoped layer of ZnTe can be doped in situ with nitrogen in excess of1×10¹⁹ cm⁻³ to produce a p+ type material. A crystallizing anneal can beapplied to the ZnTe layer(s) at an elevated temperature between about50° C. and 100° C. above the deposition temperature for a time less thanabout 3 minutes.

In an embodiment, an n-p doped heterojunction of CdTe and CdZnTe ofthickness between about 1.25 micrometers (“microns”) and 4 microns canbe deposited onto the ZnTe layer at a deposition temperature betweenabout 250° C. and 350° C. CdTe can be first doped in situ with arsenicdoping concentration in the range 1×10¹⁷ and 4×10¹⁷ cm⁻³ to produce ap-type material at a thickness between about 1 microns and 3 microns. Inan alternative embodiment, for the first 200-500 Å the arsenic dopingconcentration is between about 1×10¹⁸ and 3×10¹⁸ cm⁻³ and then rampeddown to the range 1×10¹⁷ to 4×10¹⁷ cm⁻³. CdZnTe can next be doped insitu with indium in the range 1×10¹⁸ and 4×10¹⁸ cm⁻³ to produce ann-type material at a thickness between about 0.25 microns and 1 microns.The Cd_(x)Zn_((1-x))Te is compositionally graded from x=1 down to an xvalue between about 0.6 and 0.75. A crystallizing anneal can be appliedto the CdTe/CdZnTe layers at an elevated temperature between about 50°C. and 100° C. above the deposition temperature and under Cdoverpressure for a time less than about 3 minutes. The anneal can beperformed more than once during the deposition of the layers for a totaltime less than about 10 minutes at thickness steps between about 0.4microns and 0.6 microns, followed by a return to the depositiontemperature and continuation of the deposition.

In an embodiment, a second, high doped CdZnTe layer is deposited ontothe first CdZnTe layer with thickness less than or equal to about 200 Åat a deposition temperature between about 250° C. and 350° C. The secondCdZnTe layer can be doped with indium between about 1×10¹⁹ and 1×10²⁰cm⁻³ to produce an n+ type, ohmic material for metal contact.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In another embodiment of the device structure, an optional ZnTe or MgTebuffer layer of thickness less than or equal to about 200 Å can bedeposited onto the superstrate at a deposition temperature between about250° C. and 350° C. In an embodiment, a high doped layer of ZnTe ofthickness less than about 200 Å can be deposited onto the buffer layerat a deposition temperature between about 250° C. and 350° C. The highdoped layer of ZnTe layer can be doped in situ with nitrogen in excessof 1×10¹⁹ cm⁻³ to produce a p+ type material. A crystallizing anneal canbe applied to the ZnTe layers at an elevated temperature between about50° C. and 100° C. above the deposition temperature for a time less thanabout 3 minutes.

In an embodiment, an intrinsic (undoped or very low doped) CdTe (i-CdTe)layer of thickness between about 1.0 micrometer (“micron”) and 1.5microns can be deposited onto the ZnTe layer at a deposition temperaturebetween about 250° C. and 350° C. A crystallizing anneal can be appliedto the i-CdTe layer at an elevated temperature between about 50° C. and100° C. above the deposition temperature and under Cd overpressure for atime less than about 3 minutes. The anneal can be performed more thanonce during the deposition of the layer for a total time less than about10 minutes at thickness steps between about 0.4 microns and 0.6 microns,followed by a return to the deposition temperature and continuation ofthe deposition.

In an embodiment, a high doped Cd_(x)Zn_((1-x))Te layer is depositedonto the i-CdTe layer with thickness less than or equal to about 200 Åat a deposition temperature between about 250° C. and 350° C. The CdZnTelayer can be doped with indium between about 1×10¹⁹ and 1×10²⁰ cm⁻³ toproduce an n+ type, ohmic material for metal contact. In a preferredembodiment x=1.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In yet another embodiment of the device structure, an optional ZnTe orMgTe buffer layer of thickness less than or equal to about 200 Å can bedeposited onto the superstrate at a deposition temperature between about250° C. and 350° C. In an embodiment, a high doped layer of CdTe ofthickness less than about 200 Å can be deposited onto the buffer layerat a deposition temperature between about 250° C. and 350° C. The highdoped layer of CdTe layer can be doped in situ with indium in excess of1×10¹⁹ cm⁻³ to produce a n+ type material. A crystallizing anneal can beapplied to the ZnTe (or MgTe)/CdTe layers at an elevated temperaturebetween about 50° C. and 100° C. above the deposition temperature for atime less than about 3 minutes and under Cd overpressure for the CdTelayer.

In an embodiment, an n-p doped heterojunction of CdTe first and CdZnTesecond of total thickness between about 1.25 micrometers (“microns”) and4 microns can be deposited onto the CdTe layer at a depositiontemperature between about 250° C. and 350° C. The CdTe layer can bedoped in situ with indium in the range 1×10¹⁷ and 4×10¹⁷ cm⁻³ to producean n-type material at a thickness between about 1 microns and 3 microns.CdZnTe can next be doped in situ with arsenic in the range 0.5×10¹⁸ and1×10¹⁸ cm⁻³ to produce a p-type material at a thickness between about0.25 microns and 1 microns. The Cd_(x)Zn_((1-x))Te is compositionallygraded from x=1 down to an x value between about 0.6 and 0.75. Acrystallizing anneal can be applied to the CdTe/CdZnTe layers at anelevated temperature between about 50° C. and 100° C. above thedeposition temperature under Cd overpressure for a time less than about3 minutes. The anneal can be performed more than once during thedeposition of the layers for a total time less than about 10 minutes atthickness steps between about 0.4 microns and 0.6 microns, followed by areturn to the deposition temperature and continuation of the deposition.

In an embodiment, a second, high doped Cd_(x)Zn_((1-x))Te layer isdeposited onto the first CdZnTe layer with thickness less than or equalto about 200 Å at a deposition temperature between about 250° C. and350° C. The second CdZnTe layer can be doped with arsenic between about1×10¹⁸ and 5×10¹⁸ cm⁻³ to produce a p+ type, ohmic material for metalcontact. In an alternative embodiment x=0 (ZnTe) and the dopant isnitrogen between about 1×10¹⁹ and 1×10²⁰ cm⁻³ to produce a p+ type,ohmic material for metal contact.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In another embodiment of the device structure, an optional ZnTe or MgTebuffer layer of thickness less than or equal to about 200 Å can bedeposited onto the superstrate at a deposition temperature between about250° C. and 350° C. In an embodiment, a high doped layer of CdTe ofthickness less than about 200 Å can be deposited onto the buffer layerat a deposition temperature between about 250° C. and 350° C. The highdoped layer of CdTe layer can be doped in situ with indium in excess of1×10¹⁹ cm⁻³ to produce a n+ type material. A crystallizing anneal can beapplied to the ZnTe (or MgTe)/CdTe layers at an elevated temperaturebetween about 50° C. and 100° C. above the deposition temperature for atime less than about 3 minutes and under Cd overpressure for the CdTelayer.

In an embodiment, an intrinsic (undoped or very low doped) CdTe (i-CdTe)layer of thickness between about 1.0 micrometers (“microns”) and 1.5microns can be deposited onto the CdTe layer at a deposition temperaturebetween about 250° C. and 350° C. A crystallizing anneal can be appliedto the i-CdTe layer at an elevated temperature between about 50° C. and100° C. above the deposition temperature under Cd overpressure for atime less than about 3 minutes. The anneal can be performed more thanonce during the deposition of the layer for a total time less than about10 minutes at thickness steps between about 0.4 microns and 0.6 microns,followed by a return to the deposition temperature and continuation ofthe deposition.

In an embodiment, a high doped Cd_(x)Zn_((1-x))Te layer is depositedonto the i-CdTe layer with thickness less than or equal to about 200 Åat a deposition temperature between about 250° C. and 350° C. The CdZnTelayer can be doped with arsenic between about 1×10¹⁸ and 5×10¹⁸ cm⁻³ toproduce a p+ type, ohmic material for metal contact. In an alternativeembodiment x=0 (ZnTe) and the dopant is nitrogen between about 1×10¹⁹and 1×10²⁰ cm⁻³ to produce a p+ type, ohmic material for metal contact.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In yet another embodiment of the device structure, i.e., a “tandem”junction structure, an optional ZnTe or MgTe buffer layer of thicknessless than or equal to about 200 Å can be deposited onto the superstrateat a deposition temperature between about 250° C. and 350° C. In anembodiment, a high doped layer of ZnTe of thickness less than about 200Å can be deposited onto the buffer layer at a deposition temperaturebetween about 250° C. and 350° C. The high doped layer of ZnTe layer canbe doped in situ with nitrogen in excess of 1×10¹⁹ cm⁻³ to produce a p+type material. A crystallizing anneal can be applied to the ZnTe layersat an elevated temperature between about 50° C. and 100° C. above thedeposition temperature for a time less than about 3 minutes.

In an embodiment, an n-p doped higher energy band-gap (visible)homojunction of CdZnTe of thickness between about 0.5 micrometers(“microns”) and 1.25 microns can be deposited onto the ZnTe layer at adeposition temperature between about 250° C. and 350° C. CdZnTe can befirst doped in situ with arsenic in the range 1×10¹⁷ and 4×10¹⁷ cm⁻³ toproduce a p-type material at a thickness between about 0.25 microns and0.75 microns. In an alternative embodiment, for the first 200-500 Å thearsenic doping concentration is between about 1×10¹⁸ and 3×10¹⁸ cm⁻³ andthen ramped down to the range 1×10¹⁷ to 4×10¹⁷ cm⁻³. CdZnTe can next bedoped in situ with indium in the range 1×10¹⁸ and 4×10¹⁸ cm⁻³ to producean n-type material at a thickness between about 0.25 microns and 0.5microns. A crystallizing anneal can be applied to the CdZnTe layers atan elevated temperature between about 50° C. and 100° C. above thedeposition temperature and under Cd and optionally Zn overpressure for atime less than about 3 minutes. The anneal can be performed more thanonce during the deposition of the layers for a total time less thanabout 10 minutes at thickness steps between about 0.4 microns and 0.6microns, followed by a return to the deposition temperature andcontinuation of the deposition.

In an embodiment, a high doped tunneling junction (p-n junction) isdeposited to provide electrical connection between the higher energyband gap n-p CdZnTe junction and a lower energy band gap n-p CdTejunction disposed over the higher energy band gap n-p CdZnTe layer. Thep-n tunneling junction comprises a high doped n+ CdZnTe layer of thesame composition as the n-type CdZnTe layer below, with a thickness lessthan about 100 Å and indium dopant concentration greater than or equalto about 5×10¹⁹ cm⁻³. The tunneling junction further comprises a highdoped p+ CdTe over the n+ CdZnTe layer, the p+ CdTe layer having athickness less than about 50 Å and an arsenic dopant concentrationgreater than or equal to about 3×10¹⁸ cm⁻³.

In an embodiment, an n-p doped lower energy band gap (near infrared)homojunction of CdTe of thickness between about 1.0-1.5 microns isdeposited onto the tunneling junction at a deposition temperaturebetween about 250° C.-350° C. During formation of the n-p doped CdTelayer, CdTe can first be doped in situ with arsenic in the range ofabout 1×10¹⁷-4×10¹⁷ cm⁻³ to produce a p-type material at a thicknessbetween about 0.75-1 micron, and next doped in situ with indium in therange 1×10¹⁸-4×10¹⁸ cm⁻³ to produce an n-type material at a thicknessbetween about 0.25-0.5 micron. An optional crystallizing anneal can beapplied to the CdTe layers at an elevated temperature between about 50°C. and 100° C. above the deposition temperature and under Cdoverpressure for a time less than about 3 minutes. The anneal can beperformed more than once during the deposition of the layers for a totaltime less than about 10 minutes at thickness steps between about 0.4microns and 0.6 microns, followed by a return to the depositiontemperature and continuation of the deposition.

In an embodiment, a second, high doped CdTe layer is deposited onto thefirst CdTe layer with thickness less than or equal to about 200 Å at adeposition temperature between about 250° C. and 350° C. The second CdTelayer can be doped with indium between about 1×10¹⁹ and 1×10²⁰ cm⁻³ toproduce an n+ type, ohmic material for metal contact.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In another embodiment of the device structure, i.e., another “tandem”junction structure, an optional ZnTe or MgTe buffer layer of thicknessless than or equal to about 200 Å can be deposited onto the superstrateat a deposition temperature between about 250° C. and 350° C. In anembodiment, a high doped layer of CdZnTe of thickness less than about200 Å can be deposited onto the buffer layer at a deposition temperaturebetween about 250° C. and 350° C. The high doped layer of CdZnTe can bedoped with indium between about 1×10¹⁹ and 1×10²⁰ cm⁻³ to produce an n+type, ohmic material. A crystallizing anneal can be applied to theCdZnTe layers at an elevated temperature between about 50° C. and 100°C. above the deposition temperature under Cd overpressure for a timeless than about 3 minutes.

In an embodiment, an n-p doped higher energy band-gap (visible)homojunction of CdZnTe of thickness between about 0.5 micrometers(“microns”) and 1.25 microns can be deposited onto the CdZnTe layer at adeposition temperature between about 250° C. and 350° C. CdZnTe can befirst doped in situ with indium in the range 1×10¹⁷ and 4×10¹⁷ cm⁻³ toproduce an n-type material at a thickness between about 0.25 microns and0.75 microns. CdZnTe can next be doped in situ with arsenic in the range0.5×10¹⁸ and 1×10¹⁸ cm⁻³ to produce a p-type material at a thicknessbetween about 0.25 microns and 0.5 microns. A crystallizing anneal canbe applied to the CdZnTe layers at an elevated temperature between about50° C. and 100° C. above the deposition temperature under Cd andoptionally Zn overpressure for a time less than about 3 minutes. Theanneal can be performed more than once during the deposition of thelayers for a total time less than about 10 minutes at thickness stepsbetween about 0.4 microns and 0.6 microns, followed by a return to thedeposition temperature and continuation of the deposition.

In an embodiment, a high doped tunneling junction (p-n junction) isdeposited to provide electrical connection between the higher energyband gap n-p CdZnTe junction and a lower energy band gap n-p CdTejunction disposed over the higher energy band gap n-p CdZnTe layer. Thep-n tunneling junction comprises a high doped p+ CdZnTe layer of thesame composition as the p-type CdZnTe layer below, with a thickness lessthan about 50 Å and arsenic dopant concentration greater than or equalto about 3×10¹⁸ cm⁻³. The tunneling junction further comprises a highdoped n+ CdTe over the p+ CdZnTe layer, the n+ CdTe layer having athickness less than about 100 Å and an indium dopant concentrationgreater than or equal to about 5×10¹⁹ cm⁻³.

In an embodiment, an n-p doped lower energy band gap (near infrared)layer of CdTe of thickness between about 1.0-1.5 microns is depositedonto the tunneling junction at a deposition temperature between about250° C.-350° C. During formation of the n-p doped CdTe layer, CdTe layercan be first doped in situ with indium in the range of about1×10¹⁷-4×10¹⁷ cm⁻³ to produce an n-type material at a thickness betweenabout 0.75-1 micron, and next doped in situ with arsenic in the range0.5×10¹⁸-1×10¹⁸ cm⁻³ to produce a p-type material at a thickness betweenabout 0.25-0.5 micron. An optional crystallizing anneal can be appliedto the CdTe layers at an elevated temperature between about 50° C. and100° C. above the deposition temperature under Cd overpressure for atime less than about 3 minutes. The anneal can be performed more thanonce during the deposition of the layers for a total time less thanabout 10 minutes at thickness steps between about 0.4 microns and 0.6microns, followed by a return to the deposition temperature andcontinuation of the deposition.

In an embodiment, a second, high doped CdTe layer is deposited onto thefirst CdTe layer with thickness less than or equal to about 200 Å at adeposition temperature between about 250° C. and 350° C. The second CdTelayer can be doped with arsenic between about 1×10¹⁸ and 5×10¹⁸ cm⁻³ toproduce a p+ type, ohmic material for metal contact.

In an embodiment, a metal contact is deposited onto the photovoltaiccell in situ with thickness on the order of 10,000 Å. The photovoltaiccell deposited (or formed) on the superstrate can be transferred invacuum from the primary semiconductor deposition chamber to a secondchamber for metal deposition under vacuum.

In an aspect of the invention, a photovoltaic device is provided, the PVdevice comprising a first layer comprising tellurium (Te) and cadmium(Cd) or zinc (Zn) over a substrate; a second layer comprising Cd and Teover the first layer; and a third layer comprising Cd, Zn and Te overthe second layer.

In another aspect of the invention, a PV device is provided, the PVdevice comprising a p-type ZnTe layer over a substrate; a p-type CdTelayer over the p-type ZnTe layer; a first n-type CdZnTe layer over thep-type CdTe; and a second n-type CdZnTe layer over the first n-typeCdZnTe layer.

In yet another aspect of the invention, a PV device is provided, the PVdevice comprising an n-type layer including Cd and Te; an intrinsic CdTelayer over the n-type layer; and a p-type layer including Te and one ormore of Cd and Zn over the intrinsic CdTe layer. In an embodiment, thePV device further comprising a substrate below the n-type layer. In analternative embodiment, the PV device comprises a substrate above thep-type layer.

In still another aspect of the invention, a PV device is provided, thePV device comprising a first n-type CdTe layer over a substrate; asecond n-type CdTe layer over the first n-type CdTe layer; a firstp-type CdZnTe layer over the second n-type CdTe layer; and a secondp-type CdZnTe layer over the first p-type CdZnTe layer.

In still another aspect of the invention, a photovoltaic device isprovided, the PV device comprising an intrinsic CdTe layer between ann-type layer having Cd and Te and a p-type layer having Zn and Te,wherein the n-type layer is disposed below the intrinsic CdTe layer. Inan embodiment, the PV device comprises a substrate or superstrate belowthe n-type layer. In an alternative embodiment, the PV device comprisesa substrate or superstrate above the p-type layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 shows a “reverse” p-n junction solar cell structure, inaccordance with an embodiment of the invention;

FIG. 2 shows a “reverse” p-intrinsic-n solar cell structure, inaccordance with an embodiment of the invention;

FIG. 3 shows an n-p junction solar cell structure, in accordance with anembodiment of the invention; and

FIG. 4 shows an n-intrinsic-p junction solar cell structure, inaccordance with an embodiment of the invention;

FIG. 5 shows a “reverse” tandem junction solar cell structure, inaccordance with an embodiment of the invention; and

FIG. 6 shows a tandem junction solar cell structure, in accordance withan embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While various embodiments of the invention have been shown and describedherein, it will be obvious to those skilled in the art that suchembodiments are provided by way of example only. Numerous variations,changes, and substitutions will now occur to those skilled in the artwithout departing from the invention. It should be understood thatvarious alternatives to the embodiments of the invention describedherein may be employed in practicing the invention.

In current thin film photovoltaic cells, such as CdTe or CIGS, a CdS“window” layer is used because it is an intrinsically n-type material.Because current process technologies used in production do not providethe capability of doping photovoltaic structures in situ (i.e., realtime in the deposition chamber), those of skill in the art use amaterial with high intrinsic n-type doping, such as CdS, to define then-type layer of the p-n junction. But there are limitations associatedwith using CdS. For example, CdS (at a CdS/CdTe interface) can reduceuseable electrical current by absorbing incoming photons, which in turncreate charge carriers that contribute very little, if at all, to theelectrical current of the diode. In some cases, this problem is due to acombination of a band gap barrier between the CdS/CdTe layers and largerecombination rates at a low quality CdS/CdTe interface layer. Inovercoming these limitations, one approach is to reduce the thickness ofthe CdS light absorbing layer as much as possible to limit the amount ofincoming light that is absorbed in this “dead layer.” But below about100 nanometers, the CdS layer has pinholes and non-uniformities thatdegrade device performance.

In various embodiments, methods for forming cadmium telluride (CdTe)thin film solar cell structures are provided. Methods of embodimentsprovide for forming high quality CdTe thin films at high depositionrates. CdTe thin film structures of preferable embodiments can providefor high power efficiency conversion in solar cell (also “photovoltaiccell” or “photovoltaic” herein) devices.

Methods of preferable embodiments are suitable for forming solar panelsusing molecular beam epitaxy (“MBE”) at high deposition rates andpolycrystalline deposition modes, while still providing the advantagesof doping, composition and uniformity control of MBE. Methods of variousembodiments enable formation of single and tandem junction solar cellstructures having uniform compositions, longer lifetime, and largergrain sizes, which provide for enhanced device performance.

In preferable embodiments, doping of structural layers of solar celldevices with shallow donors and acceptors is performed in situ (i.e.,during deposition) during epitaxial growth of solar cell devicestructural layers. Conventional chemical vapor deposition techniques(other than MBE) suffer from low solubility issues with the shallowlevel donors/acceptors or difficulty with complete ionization for deeperlevel donors/acceptors. By doping the structure in situ the solubilityissues are overcome and hence the technique allows the use of theshallow donor/acceptors to provide high doping levels, necessary tobuild improved performance solar cells. This advantageously reduces, ifnot eliminates, interstitial or intrinsic (defect) dopants by providingsubstitutional dopants. Substitutional dopants can provide for morestable solar cell devices because of their much lower diffusion comparedto interstitial dopants. MBE methods of preferable embodiments canadvantageously provide for forming high quality thin film solar celldevices with higher power efficiency in relation to prior art thin filmsolar cell devices.

Methods and structure of embodiments of the invention can providephotovoltaic devices with improved short circuit current (Jsc), opencircuit voltage (Voc), and fill factor (FF) in relation to prior artthin film photovoltaic devices. In one embodiment, a “reverse” p-njunction (“reverse” from the point of view of the current technologieswhich deposit the n-type portion of the junction on the superstrate andfollow with deposition of the p-type portion of the junction; in thisembodiment, that order is reversed with the p-type portion deposited onthe superstrate first, followed by the n-type portion of the junctionwhich now makes contact to the backside metallization) photovoltaicdevice having a power efficiency between about 18% and 22% isachievable. In another embodiment, a “reverse” n-intrinsic-p junctionphotovoltaic device having a power efficiency between about 18% and 22%is achievable. In another embodiment, an n-p junction photovoltaicdevice having a power efficiency between about 18% and 22% isachievable. In another embodiment, an n-intrinsic-p junctionphotovoltaic device having a power efficiency between about 18% and 22%is achievable. In another embodiment, a tandem junction photovoltaicdevice having a power efficiency between about 24% and 28% isachievable.

Thin film solar cell structures of preferable embodiments can be formedin one or more in-line vacuum chambers configured for molecular beamepitaxy (“MBE”). The one or more vacuum chambers may include a primarymolecular beam (“MB”) chamber and one or more in-line auxiliary (orsecondary) chambers. The vacuum chambers can be maintained under mediumvacuum (1×10⁻⁶ to 1×10⁻⁵ torr, or 1×10⁻⁷ to 1×10⁻⁶ torr) or high vacuum(1×10⁻⁸ to 1×10⁻⁷ torr) during operation with the aid of a pumpingsystem comprising one or more of an ion pump, a turbomolecular (“turbo”)pump, a cryopump and a diffusion pump. The pumping system may alsoinclude one or more “backing” pumps, such as mechanical or dry scrollpumps. Vacuum chambers of preferable embodiments may include a maindeposition chamber for forming various device structures, in addition toauxiliary chambers for forming additional device structures, such as,e.g., backside metal contact (“metallization”) and solar panel lasercell scribing. In an alternative embodiment, multiple in-line vacuumchambers can be arranged to provide particular layer depositions of theoverall device structure, with increases in overall through-put.Molecular beam systems of preferable embodiments may comprise one ormore vacuum chambers, pumping systems and a computer system configuredto control vacuum chamber pressure, substrate temperature, materialsource temperatures, and various parameters (e.g., source partialpressure, source flux, deposition time, exposure time) associated withthe deposition of solar cell device structures.

This deposition method applies to any vacuum deposition technique thatcan (i) control the doping as the material is grown (in situ), (ii)control the thicknesses of different compositional layers, (iii) controlthe deposition rate during growth, and (iv) control the compositionalchange from one layer to another layer by varying the ratio of elementsin the composition. This includes, but is not limited to, conventional(solid phase) MBE, gas phase MBE (GPMBE), and metalorganic chemicalvapor deposition (MOCVD), and any other vapor deposition that meets theabove requirements, especially requirements (i)-(iii). In a preferableembodiment, the MBE approach is employed.

In embodiments of the invention, methods, apparatuses and/or structuresprovide for the following: (i) polycrystalline growth at high depositionrate; (ii) cell architectures that remove the problematic CdS “window”layer; (iii) deposition with complete doping control, in situ, tooptimize the cell structure with respect to doping concentrations; (iv)compositional grading of heterojunction layers to optimize the cellstructure by significant reduction in interface recombination sites; (v)the capability to heavily dope material grown over a superstrate (orsubstrate), in situ, near front and back contacts to create one or morelow ohmic contacts; (vi) providing passivation of grain boundaries, insitu, by heavily doping the grain boundaries to repel minority carriersfrom the boundary recombination sites; and (vii) providing completedeposition rate control to allow deposition interruption forcrystallizing anneals, in situ, and allowing highly reduced growth ratefor the initial seed layers in order to optimize grain size. Inembodiments, capabilities (iii) and (iv) above, when combined, allow forcomplete control over the position of the junction for theheterostructure for optimized performance, which is achieved by placingthe junction substantially near the narrower band-gap material.

As used herein, “n-type layer” refers to a layer having an n-typechemical dopant and “p-type layer” refers to a layer having a p-typechemical dopant. N-type layers and p-type layers can have othermaterials in addition to n-type and p-type dopants. For example, ann-type CdTe layer is a layer formed of Cd and Te that is also chemicallydoped n-type. As another example, a p-type ZnTe layer is a layer havingZn and Te that is also chemically doped p-type.

Reverse p-n and p⁺-Intrinsic-n⁺ Junction Solar Cell Structures

In an aspect of the invention, a “reverse” p-n junction solar cell (orphotovoltaic) device is grown by MBE on a superstrate, with or without atransparent conductive oxide (TCO). In a preferable embodiment, thehighly doped front layer of the device structure serves as the frontside low ohmic contact and a TCO coating is unnecessary since depositioncan occur directly onto the bare glass superstrate. The successivesemiconductor layers grown provide, in sequence: an optional thin bufferlayer; a thin, high doped p-type, low ohmic contact layer; a p-njunction; a thin, high doped n-type, low ohmic layer; and an optionallow ohmic “semimetal” contact (e.g., HgTe or SbTe). A metal contact isprovided at the backside of the structure. The metal contact, along withthe concomitant laser cell scribing, may be formed via in situmetallization and scribing.

In some embodiments, the solar cell structure may have at least 3 layersof different compound semiconductor materials. In some instances, thosesemiconductor materials may comprise ZnTe, MgTe, x-gradedCd_(x)Zn_(1-x)Te, and CdTe. The reverse p-n junction solar cellstructure may optionally include an SbTe (Sb₂Te₃) or HgTe layer forproviding contact to a metal contact at the backside of the p-n junctionsolar cell structure (also “the structure” herein).

With reference to FIG. 1, a reverse p-n junction photovoltaic (“PV”)cell (also “solar cell” herein) structure comprises a p-type (i.e.,doped p-type) CdTe layer over a superstrate and an n-type (i.e., dopedn-type) Cd_(x)Zn_(1-x)Te layer over the p-type CdTe layer. The n-typeCd_(x)Zn_(1-x)Te layer and the p-type CdTe layer define a p-nheterojunction (or structure) of the “reverse” p-n junction PV cell. Inan embodiment, the p-n layer is formed of polycrystalline CdTehomojunction, with ‘x’ equal to 1, or CdTe/Cd_(x)Zn_(1-x)Teheterojunction with ‘x’ between about 0.60 and 0.75. The p-type CdTelayer and the n-type Cd_(x)Zn_(1-x)Te layer define the light-absorbinglayers of the PV cell with the p-type CdTe layer thickness sufficient toabsorb a large majority (greater than about 90%) of the incoming light.

The reverse p-n junction PV cell may include an optional, thin intrinsic(i.e., undoped or very low doped) resistive ZnTe or MgTe layer betweenthe superstrate (with or without TCO) and the p-type CdTe layer. A thin,high doped p-type ZnTe (i.e., p+ ZnTe) layer may be provided between theoptional resistive ZnTe layer and the p-type CdTe layer.

The reverse p-n junction PV cell may further include a metal contactinglayer over the n-type Cd_(x)Zn_(1-x)Te layer. To improve electricalcontact between the metal contact and the n-type Cd_(x)Zn_(1-x)Te layer,a thin, high doped n-type Cd_(x)Zn_(1-x)Te (i.e., n+ Cd_(x)Zn_(1-x)Te)layer may be provided between the n-type Cd_(x)Zn_(1-x)Te layer and themetal contact. For further improvement in electrical contact between themetal contact and the n-type Cd_(x)Zn_(1-x)Te layer, a thin, SbTe orHgTe layer may be provided between the n-type thin, high dopedCd_(x)Zn_(1-x)Te layer (n+ Cd_(x)Zn_(1-x)Te) and the metal contact, or,alternative, between the n-type Cd_(x)Zn_(1-x)Te layer and the metalcontact.

With continued reference to FIG. 1, the reverse p-n junction solar cellcan further include an antireflective (“AR”) coating layer at thesuperstrate frontside (where light enters the reverse p-n junction solarcell) and/or between the superstrate and the transparent conductiveoxide, if present, or the first structure layer, if absent. The AR layercan aid in minimizing reflection of light incident on the reverse p-njunction solar cell. The reverse p-n junction solar cell can furtherinclude an antireflective (“AR”) coating layer (or layers on each sideof the superstrate) that is configured to reflect certain wavelengths oflight and absorb certain wavelengths of light so as to provide anesthetically appealing custom color to the visible surface of the solarpanel (i.e., solar panel art or architectural appeal).

With continued reference to FIG. 1, one or more electrical contacts areprovided at the frontside (superstrate). In an embodiment, an etch isused to access the frontside (superstrate) transparent conductive oxide,if present, or the high doped contact layer, if absent, to form theelectrical contact at the frontside. In another embodiment, laserscribing is used to access the frontside (superstrate) conducting layerto form the electrical contact at the frontside.

With reference to FIG. 2, in an alternative embodiment, an intrinsic (orvery low doped) CdTe (i.e., i-CdTe) layer is provided on the high dopedp+ ZnTe layer, and the high doped n+ Cd_(x)Zn_(1-x)Te layer is formedover the i-CdTe layer. In such a case, the i-CdTe partially defines thep-intrinsic-n CdTe structure of a p-intrinsic-n junction solar celldevice. The i-CdTe layer can be formed of polycrystalline CdTe. In apreferred embodiment, the i-CdTe layer has a thickness between about 1.0and 1.5 microns. The i-CdTe layer can be deposited at a depositiontemperature between about 200° C. and about 400° C., or between about250° C. and about 350° C. Following formation of the i-CdTe(light-absorbing) layer, an optional crystallizing and grain boundarypassivating anneal can be performed at a temperature difference betweenabout 25° C. and 125° C., or between about 50° C. and 100° C. above thei-CdTe deposition temperature. The crystallizing and grain boundarypassivating anneal can be performed for a time period less than or equalto about 10 minutes, or less than or equal to about 5 minutes, or lessthan or equal to about 3 minutes.

In a preferable embodiment, the crystallizing and grain boundarypassivating anneal is performed under a Cd overpressure. In such a case,all other sources of material flux are closed off during the passivatinganneal. In another embodiment, the crystallizing and grain boundarypassivating anneal is performed under a Cd and Zn overpressure. In sucha case, all other sources of material flux are closed off during thisanneal. In an embodiment, the crystallizing and grain boundarypassivating anneal is performed more than once and at predeterminedintervals during formation of the i-CdTe light-absorbing layer. Thecrystallizing and grain boundary passivating anneal can be performed ati-CdTe light-absorbing layer thickness steps between about 0.2 micronsand about 0.8 micron, or between about 0.4 microns and about 0.6microns, for a time period less than or equal to about 3 minutes, orless than or equal to about 2 minutes, or less than or equal to about 1minute per anneal, followed by a return (of the superstrate) to thedeposition temperature and continuation of the deposition of the i-CdTelight absorbing layer.

One or more of the layers discussed herein, in relation to variousembodiments of the invention, may be optional. In some embodiments, thelayers may be provided as described, while in other embodiments somevariation in sequence may be provided (e.g., switching the sequence oflayers CdTe/CdZnTe for the p-n heterojunction). Neighboring layers thatdiffer in compositional structure by addition (and/or removal) of anelement (e.g., a CdTe adjacent a ZnTe layer, or a CdTe layer adjacent aCd_(x)Zn_(1-x)Te layer) may be graded between the two compositions byvarying the mole fraction ‘x’ to ameliorate band-gap barriers that arisefrom directly depositing two different band-gap materials next to eachother. This grading will occur over a thickness between about 0.1microns and 0.5 microns.

With reference to FIG. 1, in a preferable embodiment, a reverse p-njunction solar cell structure is shown. The reverse p-n junction solarcell structure may comprise a highly resistive, thin film ZnTe or MgTebuffer layer on a superstrate (“Glass superstrate, tempered,” asillustrated), and a thin, high doped p-type ZnTe layer over the thinresistive ZnTe (or MgTe) buffer layer. The superstrate can be formed ofa semiconductor material or an amorphous material such as, e.g.,standard soda lime glass. An optional transparent conductive oxide (TCO)layer can be provided over the superstrate to provide an electricalfront contact. Alternatively a thin metal foil substrate can be usedwith the cell structure embodiments grown in reverse order so theincoming light continues to see the same layer sequence as with asuperstrate. The final deposition layer in this sequence must be atransparent conductive oxide deposited in an in-line chamber next to theprimary deposition chamber or the high doped contact layer of the devicestructure itself. The highly resistive buffer layer can have a thicknessless than or equal to about 300 Å, or less than or equal to about 200 Å,or less than or equal to about 100 Å. The high doped p-type ZnTe layercan have a thickness less than or equal to about 300 Å, or less than orequal to about 200 Å, or less than or equal to about 100 Å. The twolayers can each be deposited on the superstrate at a depositiontemperature between about 200° C. and about 400° C., or between about250° C. and about 350° C. In a preferable embodiment, the two layers areformed via molecular beam epitaxy (“MBE”) at a growth rate about 1 Å persecond.

In a preferable embodiment, the high doped ZnTe layer is doped in situwith nitrogen to produce a p+ material layer having a nitrogen dopantconcentration between about 1×10¹⁹ cm⁻³ and about 1×10²⁰ cm⁻³.

Following formation of the two layers, an optional crystallizing annealcan be performed at a temperature difference between about 25° C. and125° C., or between about 50° C. and 100° C. above the higher layers'deposition temperature. The crystallizing anneal can be performed for atime period less than or equal to about 5 minutes, or less than or equalto about 3 minutes, or less than or equal to about 1 minute. During theanneal, all deposition sources may be closed. Following the anneal, areturn to the deposition temperature and continuation of the depositionmay commence.

After forming the buffer and high doped layers, a CdTe/Cd_(x)Zn_(1-x)Telight-absorbing layer (also “absorber layer” herein) may be grown as ap-type and n-type heterojunction (or homojunction in case x is equal to1). P-type doping can be achieved with the aid of arsenic; n-type dopingcan be achieved with the aid of indium. The p-type CdTe light absorbinglayer can have a thickness of between about 1.0 microns and about 3.0microns. The p-type CdTe light absorbing layer can be formed at adeposition temperature between about 200° C. and about 350° C., orbetween about 250° C. and about 300° C. In a preferable embodiment, theCdTe layer is doped in situ with arsenic to produce a p-type materiallayer having an arsenic activated doping concentration between about1×10¹⁷ cm⁻³ and 4×10¹⁷ cm⁻³. In an alternative embodiment, the first200-500 Å of arsenic doping concentration is between about 1×10¹⁸ to3×10¹⁸ cm⁻³ and then ramped down to the range 1×10¹⁷ to 4×10¹⁷ cm⁻³. Then-type Cd_(x)Zn_(1-x)Te layer can have a thickness between about 0.25microns and about 1 microns. The n-type Cd_(x)Zn_(1-x)Te light absorbinglayer can be formed at a deposition temperature between about 200° C.and about 350° C., or between about 250° C. and about 300° C. In apreferable embodiment, the Cd_(x)Zn_(1-x)Te layer is doped in situ withindium to produce an n-type material layer having an indium activateddoping concentration between about 1×10¹⁸ cm⁻³ and 4×10¹⁸ cm⁻³. In anembodiment, the n-type Cd_(x)Zn_(1-x)Te layer is formed immediatelyfollowing formation of the p-type CdTe layer and at the same superstratetemperature as the CdTe deposition. For instance, while forming thep-type CdTe layer by exposing the solar cell structure to a Cd source, aTe source and an As source, the As source can be closed off and an Insource and a Zn source can be immediately introduced.

Following formation of the CdTe p-type light-absorbing layer, anoptional crystallizing and grain boundary passivating anneal can beperformed at a temperature difference between about 25° C. and 125° C.,or between about 50° C. and 100° C. above the CdTe depositiontemperature. The crystallizing and grain boundary passivating anneal canbe performed for a time period less than or equal to about 10 minutes,or less than or equal to about 5 minutes, or less than or equal to about3 minutes. In an embodiment, the crystallizing and grain boundarypassivating anneal is performed under a Cd and As overpressure. Allother sources of material flux are closed off during this anneal. Inanother embodiment, the crystallizing and grain boundary passivatinganneal is performed under a Cd and Zn overpressure with an optional Asoverpressure. All other sources of material flux are closed off duringthis anneal. In an embodiment, the crystallizing and grain boundarypassivating anneal is performed more than once and at predeterminedintervals during formation of the CdTe light-absorbing layer. In such acase, the crystallizing and grain boundary passivating anneal can beperformed at CdTe light-absorbing layer thickness steps between about0.2 microns and about 0.8 micron, or between about 0.4 microns and about0.6 microns, for a time period less than or equal to about 3 minutes, orless than or equal to about 2 minutes, or less than or equal to about 1minute per anneal, and followed by a return to the depositiontemperature and continuation of the deposition of theCdTe/Cd_(x)Zn_(1-x)Te light absorbing layer.

Following formation of the Cd_(x)Zn_(1-x)Te n-type light-absorbinglayer, a thin, high doped n-type Cd_(x)Zn_(1-x)Te (n+ Cd_(x)Zn_(1-x)Te)layer can be grown between the n-type Cd_(x)Zn_(1-x)Te layer and thefinal metal contact to provide a low ohmic contact between theCd_(x)Zn_(1-x)Te n-type light absorbing layer and the metal contact.N-type doping of the n+ Cd_(x)Zn_(1-x)Te layer can be achieved with theaid of indium. The n+ Cd_(x)Zn_(1-x)Te layer can have a thickness lessthan or equal to about 300 Å, or less than or equal to about 200 Å, orless than or equal to about 100 Å. The n+ Cd_(x)Zn_(1-x)Te layer can beformed at a deposition temperature between about 200° C. and about 350°C., or between about 250° C. and about 300° C. The concentration ofn-type dopant (e.g., indium) in the n+ Cd_(x)Zn_(1-x)Te layer can bebetween about 1×10¹⁹ and 1×10²⁰ cm⁻³. In an embodiment, the depositiontemperature of the n+ Cd_(x)Zn_(1-x)Te layer is the same as thedeposition temperature of the Cd_(x)Zn_(1-x)Te n-type light-absorbinglayer.

An optional metal contact layer can provide the final contact betweenthe Cd_(x)Zn_(1-x)Te layers (light absorbing layer and high n-type dopedlayer) and the metallization of the backside of the structure. The finalmetal contact layer is formed by exposure of the cell to Sb and Tesources of flux or Hg and Te sources of flux, with all other sources ofmaterial flux closed off. The formation of this SbTe (HgTe) layer canhave a thickness less than or equal to about 300 Å, or less than orequal to about 200 Å, or less than or equal to about 100 Å. The SbTelayer can be deposited at a deposition temperature between about 200° C.and about 350° C., or between about 250° C. and about 300° C. In apreferable embodiment, the deposition temperature of the SbTe layer isthe same as the deposition temperature of the Cd_(x)Zn_(1-x)Te layers.The HgTe layer can be deposited at a deposition temperature betweenabout 150° C. and about 250° C.

The final metal contact and laser cell scribing can be formed in situ inauxiliary chambers (or secondary chambers). The auxiliary chambers arein-line with the primary MBE vacuum chamber. The primary MBE vacuumchamber may be the primary semiconductor deposition chamber. The metalcontact and concomitant cell scribing may be formed in situ bytransferring the photovoltaic device of FIG. 1 from the primary MBEvacuum chamber to the auxiliary in-line chambers under vacuum. The metalcontact layer can have a thickness between about 10,000 Å and 20,000 Å.

The structure of FIG. 1 includes a p-n junction capable of absorbinglight (such as solar light) at wavelengths from near ultraviolet (“UV”)to about 850 nm, and creating electricity by the flow of chargegenerated when the p-n junction is exposed to light. Embodiments providein situ methods for forming low ohmic metal contacts to the front andbackside of the p-n junction solar cell, high doping of the absorberlayers, passivation of the grain boundaries, compositionally-gradedheterostructures, and high accuracy control of layer thicknesses andjunction location, in order to optimize the extraction ofphoto-generated current and open circuit voltage when the absorber layerof the p-n junction solar cell is exposed to light.

The reverse p-n junction structure of FIG. 1, or as otherwise described,can be formed in a vacuum chamber configured for molecular beam epitaxy(“MBE”). The MBE chamber may be attached to one or more other vacuumchambers for forming one or more layers of the p-n junction structure.For instance, the MBE chamber may be attached to a vacuum chamberconfigured for forming the metal contact via sputtering or e-beamevaporation and a vacuum chamber configured for performing the lasercell scribing. Alternatively, multiple in-line vacuum chambers can bearranged to provide particular layer depositions of the overall devicestructure, with potential increase in overall through-put.

Formation of one or more layers of the reverse p-n junction structuremay be achieved via any MBE technique known in the art or similar highvacuum techniques that provide a free-streaming flux of elements orreactive molecules. In an embodiment, one or more layers of reverse p-njunction structures of embodiments are formed by heritage MBE, whichprovides high throughput, polycrystalline deposition while retaining thecontrol advantages of conventional MBE. The flux of elements may beadjusted to provide a deposition rate less than or equal to about 20microns/hour, or less than or equal to about 10 microns/hours, less thanor equal to about 1 microns/hour, depending on the layer beingdeposited. In a preferable embodiment, the flux of elements may beadjusted to provide a deposition rate between about 6 and 10microns/hour for the bulk p-n junction and back contact layer growthsand a deposition rate less than or equal to about 1 micron/hour for theinitial thin buffer layer and high doped p-type starting layers. MBE isused to produce a polycrystalline material structure with a totalthickness between about 1.25 micrometers (“microns”) and about 4 micronson an optically transparent superstrate, e.g., a transparent conductiveoxide-coated glass superstrate, at a deposition temperature betweenabout 200° C. and about 350° C., or between about 250° C. and about 300°C., on a superstrate area greater than or equal to about 0.36 m² (i.e.,a superstrate having a length and/or a width greater than or equal toabout 600 mm). In an embodiment, the layers are grown at the sametemperature or within 25° C. of each other. In an embodiment, the totalstructure has a thickness of about 1.25 microns. In a preferredembodiment, the superstrate area is greater than or equal to about 1 m².

n-p and n⁺-Intrinsic-p⁺ Junction Solar Cell Structures

In another aspect of the invention, an n-p junction solar cell (orphotovoltaic) device is grown by MBE on a superstrate with or without atransparent conductive oxide (TCO). In a preferable embodiment, thehighly doped front layer of the device structure serves as the frontside low ohmic contact and a TCO coating is unnecessary since depositioncan occur directly onto the bare glass superstrate. The semiconductorlayers grown in sequence over a superstrate include: an optional thinbuffer layer; a thin, high doped n-type, low ohmic contact layer; an n-pjunction; a thin, high doped p-type, low ohmic contact layer; anoptional very low ohmic “semimetal” contact, e.g., SbTe, as the finalsemiconductor layer. A metal contact is provided at the backside of thecomplete structure. The metal contact, along with the concomitant lasercell scribing, may be formed via in situ metallization and scribing.

In some embodiments, the solar cell structure may have at least threelayers of different semiconductor materials. In some embodiments, thesemiconductor materials may comprise material selected from the groupconsisting of ZnTe, MgTe, x-graded Cd_(x)Zn_(1-x)Te, and CdTe. The n-pjunction solar cell structure may optionally include an SbTe (Sb₂Te₃) orHgTe layer for providing contact to a metal contact at the backside ofthe p-n junction solar cell structure (also “the structure” herein).

With reference to FIG. 3, an n-p junction photovoltaic (“PV”) cell (also“solar cell” herein) structure comprises an n-type (i.e., doped n-type)CdTe layer over a superstrate and a p-type (i.e., doped p-type)Cd_(x)Zn_(1-x)Te layer over the n-type CdTe layer, in accordance with anembodiment of the invention. The n-type CdTe layer and the p-typeCd_(x)Zn_(1-x)Te layer define an n-p heterojunction (or structure). Thisheterojunction advantageously precludes the need for the CdS n-typelayer of prior thin film devices. In an embodiment, with ‘x’ equal to 1,the n-p layer is formed of polycrystalline CdTe homojunction. In anotherembodiment, ‘x’ is greater than 0 and less than 1, and the n-p layer isformed of a CdTe/Cd_(x)Zn_(1-x)Te heterojunction. In an embodiment, ‘x’is equal to about 0.60. In another embodiment, ‘x’ is equal to about0.75.

With continued reference to FIG. 3, the n-type CdTe layer and the p-typeCd_(x)Zn_(1-x)Te layer define the light-absorbing layers of the PV cell.The n-p junction PV cell can include an optional, thin intrinsic (i.e.,undoped or very low doped) resistive ZnTe or MgTe layer (also “bufferlayer” herein) between the superstrate and the n-type CdTe layer. Athin, high doped n-type CdTe layer (i.e., n+ CdTe) may be providedbetween the optional resistive buffer layer and the n-type CdTe layer.

The n-p junction PV cell can further include a metal contacting layerover the p-type Cd_(x)Zn_(1-x)Te layer. To improve electrical contactbetween the metal contact and the p-type Cd_(x)Zn_(1-x)Te layer, a thin,high doped p-type Cd_(x)Zn_(1-x)Te (i.e., p+ Cd_(x)Zn_(1-x)Te) layer maybe provided between the p-type Cd_(x)Zn_(1-x)Te layer and the metalcontact. To improve electrical contact between the metal contact and thep-type Cd_(x)Zn_(1-x)Te layer even further, a thin SbTe or HgTe layermay be provided between either the thin, high doped p-typeCd_(x)Zn_(1-x)Te layer (p+ Cd_(x)Zn_(1-x)Te) and the metal contact, or,alternatively, between the p-type Cd_(x)Zn_(1-x)Te layer and the metalcontact.

The n-p junction solar cell may further include an antireflective (“AR”)coating layer at the superstrate frontside (light entering side) and/orbetween the superstrate and the transparent conductive oxide (TCO), ifpresent, or the first structure layer, if absent. The AR layer can aidin minimizing reflection of light incident on the n-p junction solarcell. The n-p junction solar cell can further include an antireflective(“AR”) coating layer (or layers on each side of the superstrate) that isdesigned to advantageously reflect/absorb particular colors of the solarspectrum to create an esthetically appealing custom color to the visiblesurface of the solar panel (for solar panel art or architecturalappeal).

In an alternative embodiment (see FIG. 4), an intrinsic or substantiallylow doped CdTe (i.e., i-CdTe) layer is provided on the high doped n+CdTe layer and the high doped p+ Cd_(x)Zn_(1-x)Te layer is formed overthe i-CdTe layer. In such a case, the i-CdTe partially defines then-intrinsic-p CdTe structure of an n-intrinsic-p junction solar celldevice. The i-CdTe layer can be formed of polycrystalline CdTe. In apreferred embodiment, the i-CdTe layer has a thickness between about 1.0micron and 1.5 microns. The i-CdTe layer can be deposited at adeposition temperature between about 200° C. and about 400° C., orbetween about 250° C. and about 350° C. Following formation of thei-CdTe (light-absorbing) layer, an optional crystallizing and grainboundary passivating anneal can be performed at a temperature differencebetween about 25° C. and 125° C., or between about 50° C. and 100° C.above the i-CdTe deposition temperature. The crystallizing and grainboundary passivating anneal can be performed for a time period less thanor equal to about 10 minutes, or less than or equal to about 5 minutes,or less than or equal to about 3 minutes. In a preferable embodiment,the crystallizing and grain boundary passivating anneal is performedunder a Cd overpressure. All other sources of material flux are closedoff during this anneal. In another embodiment, the crystallizing andgrain boundary passivating anneal is performed under a Cd and Znoverpressure. All other sources of material flux are closed off duringthis anneal. In an embodiment, the crystallizing and grain boundarypassivating anneal is performed more than once and at predeterminedintervals during formation of the i-CdTe light-absorbing layer. In sucha case, the crystallizing and grain boundary passivating anneal can beperformed at i-CdTe light-absorbing layer thickness steps between about0.2 microns and about 0.8 micron, or between about 0.4 microns and about0.6 microns, for a time period less than or equal to about 3 minutes, orless than or equal to about 2 minutes, or less than or equal to about 1minute, per anneal, and followed by a return to the depositiontemperature and continuation of the deposition of the i-CdTe lightabsorbing layer.

Some of the layers discussed herein in relation to various embodimentsor aspects of the invention may be optional. In some embodiments, thelayers may be provided in the sequence described, while in otherembodiments, some variation in sequence may be provided (e.g., switchingthe sequence of the CdTe and CdZnTe layers for the p-n heterojunction).Any neighboring layers that differ in compositional structure byaddition (and/or removal) of another element (e.g., a CdTe layeradjacent a ZnTe layer, or a CdTe layer adjacent a Cd_(x)Zn_(1-x)Telayer) may be graded between the two compositions by varying the molefraction ‘x’ to ameliorate band-gap barriers that arise from directlydepositing two different band-gap materials next to each other. Thisgrading will occur over a thickness between about 0.1 microns and 0.5microns.

With reference to FIG. 3, in a preferable embodiment, an n-p junctionsolar cell structure comprises an optional, highly resistive, thin filmZnTe or MgTe buffer layer on a superstrate, and a thin, highly n-dopedCdTe layer (i.e., n+ CdTe) over the thin resistive buffer layer. Thesuperstrate can be formed of a semiconductor material or an amorphousmaterial such as, e.g., standard soda lime glass. The superstrate mayrequire an optional transparent conductive oxide (TCO) to provide theelectrical front contact. Alternatively a thin metal foil substrate canbe used with the cell structure embodiments grown in reverse order sothe incoming light continues to enter the same layer sequence as with asuperstrate. The final deposition layer in this sequence must be atransparent conductive oxide deposited in an in-line chamber next to theprimary deposition chamber or the high doped contact layer of the devicestructure itself. The buffer layer can have a thickness less than orequal to about 300 Å, or less than or equal to about 200 Å, or less thanor equal to about 100 Å. The buffer layer can be deposited over thesuperstrate at a deposition temperature between about 200° C. and about400° C., or between about 250° C. and about 350° C. In a preferableembodiment, the buffer layer is formed via molecular beam epitaxy(“MBE”) at a growth rate about 1 Å per second. The n+ CdTe layer canhave a thickness less than or equal to about 300 Å, or less than orequal to about 200 Å, or less than or equal to about 100 Å. The n+ CdTelayer can be deposited on the buffer layer at a deposition temperaturebetween about 200° C. and about 400° C., or between about 250° C. andabout 350° C. In a preferable embodiment, the CdTe layers is formed viamolecular beam epitaxy (“MBE”) at a CdTe growth rate about 1 Å persecond and at the same deposition temperature as the buffer layer.

In a preferable embodiment, the high doped n+ CdTe layer is doped insitu with indium to produce an n+ material layer having an indium dopingconcentration between about 1×10¹⁹ cm⁻³ and about 1×10²⁰ cm⁻³.

Following formation of the ZnTe (or MgTe) and n+ CdTe layers, anoptional crystallizing anneal may be performed at a temperaturedifference between about 25° C. and 125° C., or between about 50° C. and100° C. above the CdTe deposition temperature. The crystallizing annealcan be performed for a time period less than or equal to about 5minutes, or less than or equal to about 3 minutes, or less than or equalto about 1 minute. During the anneal, all deposition sources should beclosed except for the Cd source. Following the anneal, a return to thedeposition temperature and continuation of the deposition shallcommence.

After forming the ZnTe (or MgTe) and n+ CdTe layers, aCdTe/Cd_(x)Zn_(1-x)Te light-absorbing layer (also “absorber layer”herein) may be grown as an n-type and p-type heterojunction, orhomojunction in case ‘x’ equals 1. N-type doping can be achieved withthe aid of indium; p-type doping can be achieved with the aid ofarsenic. The n-type CdTe light absorbing layer can have a thickness ofbetween about 1 microns and about 3 microns. The n-type CdTe lightabsorbing layer can be formed at a deposition temperature between about200° C. and about 400° C., or between about 250° C. and about 350° C. Ina preferable embodiment, the CdTe layer is doped in situ with indium toproduce an n-type material layer having an indium activated dopingconcentration between about 1×10¹⁷ cm⁻³ and about 4×10¹⁷ cm⁻³. Thep-type Cd_(x)Zn_(1-x)Te layer can have a thickness between about 0.25microns and about 1 micron. The p-type Cd_(x)Zn_(1-x)Te light absorbinglayer can be formed at a deposition temperature between about 200° C.and about 400° C., or between about 250° C. and about 350° C. In apreferable embodiment, the Cd_(x)Zn_(1-x)Te layer is doped in situ(i.e., in the MBE chamber) with arsenic to produce a p-type materiallayer having an arsenic activated doping concentration between about5×10¹⁷ cm⁻³ and about 1×10¹⁸ cm⁻³. In a preferable embodiment, thep-type Cd_(x)Zn_(1-x)Te layer is formed immediately following formationof the n-type CdTe layer and at the same superstrate temperature as theCdTe deposition. For instance, while forming the n-type CdTe layer byexposing the solar cell structure to a Cd source, a Te source and an Insource, the In source can be closed off (or terminated) and an As sourceand a Zn source can be immediately introduced.

Following formation of the CdTe n-type light-absorbing layer, anoptional crystallizing and grain boundary passivating anneal can beperformed at a temperature difference between about 25° C. and 125° C.,or between about 50° C. and 100° C. above the CdTe depositiontemperature. The crystallizing and grain boundary passivating anneal canbe performed for a time period less than or equal to about 10 minutes,or less than or equal to about 5 minutes, or less than or equal to about3 minutes. In a preferable embodiment, the crystallizing and grainboundary passivating anneal is performed under a Cd overpressure andoptional In overpressure. All other sources of material flux are closedoff during this anneal. In an embodiment, the crystallizing and grainboundary passivating anneal is performed more than once and atpredetermined intervals during formation of the CdTe light-absorbinglayer. In such a case, the crystallizing and grain boundary passivatinganneal can be performed at CdTe light-absorbing layer thickness stepsbetween about 0.2 microns and about 0.8 micron, or between about 0.4microns and about 0.6 microns, for a time period less than or equal toabout 3 minutes, or less than or equal to about 2 minutes, or less thanor equal to about 1 minute, per anneal, and followed by a return to thedeposition temperature and continuation of the deposition of theCdTe/Cd_(x)Zn_(1-x)Te light absorbing layer.

Following formation of the Cd_(x)Zn_(1-x)Te p-type light-absorbinglayer, a thin, high doped p-type Cd_(x)Zn_(1-x)Te (p+ Cd_(x)Zn_(1-x)Te)layer can be grown between the p-type Cd_(x)Zn_(1-x)Te layer and thefinal metal contact to provide low ohmic contact between theCd_(x)Zn_(1-x)Te p-type light absorbing layer and the metal contact.P-type doping of the p+ Cd_(x)Zn_(1-x)Te layer can be achieved with theaid of arsenic. The p+ Cd_(x)Zn_(1-x)Te layer can have a thickness lessthan or equal to about 300 Å, or less than or equal to about 200 Å, orless than or equal to about 100 Å. The p+ Cd_(x)Zn_(1-x)Te layer can beformed at a deposition temperature between about 200° C. and about 400°C., or between about 250° C. and about 350° C. The concentration ofp-type dopant (e.g., arsenic) in the p+ Cd_(x)Zn_(1-x)Te layer may bebetween about 1×10¹⁸ and about 5×10¹⁸ cm⁻³. In an alternative embodimentx=0 (ZnTe) and the dopant is nitrogen at a concentration between about1×10¹⁹ and 1×10²⁰ cm⁻³ to produce a p+ type, ohmic material for metalcontact. In a preferable embodiment, the (superstrate) depositiontemperature of the p+ Cd_(x)Zn_(1-x)Te layer is the same as thedeposition temperature of the CdTe n-type light-absorbing layer.

An optional metal contact layer can provide the final contact betweenthe Cd_(x)Zn_(1-x)Te layers (light absorbing layer and high p-type dopedlayer) and the metallization of the backside of the structure. The finalmetal contact layer may be formed by exposure of the PV cell to Sb andTe sources of flux or Hg and Te sources of flux, with all other sourcesof material flux closed off. The SbTe or HgTe layer formed can have athickness less than or equal to about 300 Å, or less than or equal toabout 200 Å, or less than or equal to about 100 Å. The SbTe layer can bedeposited at a deposition temperature between about 200° C. and about400° C., or between about 250° C. and about 350° C. In an embodiment,the deposition temperature of the SbTe layer is the same as thedeposition temperature of the Cd_(x)Zn_(1-x)Te layers. The HgTe layercan be deposited at a deposition temperature between about 150° C. andabout 250° C.

The final metal contact and laser cell scribing can be formed in situ inauxiliary chambers (or secondary chambers). The auxiliary chambers maybe in-line with the primary MBE vacuum chamber. The primary MBE vacuumchamber may be the primary semiconductor deposition chamber. The metalcontact and concomitant cell scribing may be formed in situ bytransferring the photovoltaic device of FIG. 3 from the primary MBEvacuum chamber to the auxiliary in-line chambers under vacuum. The metalcontact layer may have a thickness between about 10,000 Å and 20,000 Å.

The structure of FIG. 3 includes an n-p junction capable of absorbingsolar light at wavelengths from near ultraviolet (“UV”) to about 850 nm,and creating electricity by the flow of charge generated when the n-pjunction is exposed to light. Embodiments of the invention provide insitu methods for forming low ohmic metal contacts to the front andbacksides of the n-p junction solar cell, high doping of the absorberlayers, passivation of the grain boundaries, compositionally-gradedheterostructures, and high accuracy control of layer thicknesses andjunction location, in order to optimize the extraction ofphoto-generated current and open circuit voltage when the absorber layerof the n-p junction solar cell is exposed to light.

The n-p and n-intrinsic-p junction structures of FIGS. 3 and 4 may beformed in a vacuum chamber configured for molecular beam epitaxy(“MBE”). The MBE chamber may be attached to one or more other vacuumchambers for forming one or more layers of the n-p junction structure.For instance, the MBE chamber may be attached to a vacuum chamberconfigured for forming the metal contact via sputtering or e-beamevaporation and a vacuum chamber configured for performing laser cellscribing. In an alternative embodiment, multiple in-line vacuum chamberscan be arranged to provide particular layer depositions of the overalldevice structure, with increases in overall through-put.

Formation of one or more layers of the n-p junction structure and then-intrinsic-p junction structure may be achieved via any MBE techniqueor similar high vacuum techniques that provide a free-streaming flux ofelements or reactive molecules. The flux of elements may be adjusted toprovide a deposition rate less than or equal to about 20 microns/hour,or less than or equal to about 10 microns/hours, less than or equal toabout 1 micron/hour, depending on the layer being deposited. In apreferable embodiment, the flux of elements may be adjusted to provide adeposition rate between about 6 microns/hour and about 10 microns/hourfor the bulk n-p junction and back contact layer growths and adeposition rate less than or equal to about 1 micron/hour for theinitial thin buffer and high doped n-type starting layers. MBE may beused to produce a polycrystalline material structure with a totalthickness between about 1.25 micrometers (“microns”) and about 4 micronson an optically transparent superstrate, e.g., a transparent conductiveoxide-coated glass superstrate, at a deposition temperature betweenabout 200° C. and about 400° C., or between about 250° C. and about 350°C., on a superstrate area greater than or equal to about 0.36 m² (i.e.,a superstrate having a length and/or a width greater than or equal toabout 600 mm). In an embodiment, the layers are grown at the sametemperature. In another embodiment, the layers are grown at temperatureswithin about 25° C. of each other. In an embodiment, the total structurethickness is about 1.25 microns. In an embodiment, the superstrate areais greater than or equal to about 1 m².

Tandem Solar Cell Structure

In another aspect of the invention, a double “reverse” p-n junction(also “tandem junction” and “dual junction” herein) solar cell device(also “photovoltaic device” herein) is provided having at least threelayers of semiconductor materials, including ZnTe, MgTe,Cd_(x)Zn_((1-x))Te and CdTe. The tandem junction solar cell structuremay optionally include an SbTe or HgTe layer for providing contact to ametal contact at the backside of the dual p-n junction solar cellstructure (also “the structure” herein). In a preferable embodiment, thestructure is deposited via MBE on a superstrate with or without atransparent conductive oxide (TCO) layer. In a preferable embodiment,the highly doped front layer of the device structure serves as the frontside low ohmic contact and a TCO coating is unnecessary since depositioncan occur directly onto the bare glass superstrate. The successivesemiconductor layers grown in sequence on a superstrate include: anoptional thin buffer layer; a thin, high doped p-type, low ohmic contactlayer; a higher energy band-gap p-n junction; an n-p tunneling junction(also “tunnel junction” herein); a lower energy band-gap p-n junction; athin, high doped n-type, low ohmic layer; and an optional substantiallylow ohmic semimetal contact, e.g., HgTe or SbTe, as the finalsemiconductor layer. A metal contact is provided at the backside of thecomplete structure. The metal contact, along with the concomitant lasercell scribing, may be formed via in situ metallization and scribing. Thetandem junction solar cell device of preferable embodiments may beconfigured to absorb light in the wavelength range of near ultraviolet(“UV”) to near infrared (“IR”) to generate electricity.

With reference to FIG. 5, in an embodiment, the tandem junction solarcell device may comprise an optional, highly resistive, substantiallythin ZnTe or MgTe buffer layer grown on an optional transparentconductive oxide (TCO) formed over a superstrate, such as, e.g., astandard soda lime glass substrate. This is followed by forming asubstantially thin, high doped p-type ZnTe (p+ZnTe) layer for providingohmic contact to the superstrate. A primary light absorbing structurecomprising a CdZnTe light absorbing structure and a CdTe light absorbingstructure is grown over the p+ ZnTe layer. In a preferable embodiment,the CdZnTe light absorbing structure comprises a p-type CdZnTe layer andan n-type CdZnTe layer (i.e., it is a p-n CdZnTe layer); the CdTe lightabsorbing structure comprises a p-type CdTe layer and an n-type CdTelayer (i.e., it is a p-n CdTe layer). The CdZnTe structure has a highenergy band gap that absorbs ultraviolet to visible light; longerwavelength visible light and near infrared light are passed to the CdTestructure, which has a low energy band gap (lower energy band gap thanthe CdZnTe structure). To provide an electrical connection between thetwo p-n junctions a tunneling n-p junction is provided with asubstantially thin, high doped n-type CdZnTe (n+ CdZnTe) layer formedover the first (higher energy band gap) light-absorbing p-n CdZnTe layerfollowed by a substantially thin, high doped p-type CdTe (p+CdTe) layerfollowed by deposition of the second (lower energy band gap)light-absorbing p-n CdTe layer. A thin, high doped n-type CdTe (n+ CdTe)layer is deposited over the final p-n CdTe light absorbing layer toprovide final ohmic contact to a metal contact at the backside of thetandem junction solar cell device.

Next, an optional SbTe or HgTe layer may be provided on the n+ CdTelayer to enhance the ohmic contact to the metal contact. The SbTe orHgTe layer may be formed by deposition of elemental Sb and Te orelemental Hg and Te. The tandem junction solar cell can further includean antireflective (“AR”) coating layer at the superstrate frontside(light entering side) and/or between the superstrate and the transparentconductive oxide, if present, or the first structure layer, if absent.The AR layer can aid in minimizing reflection of light incident on thetandem junction solar cell. The tandem junction solar cell can furtherinclude an antireflective (“AR”) coating layer (or layers on each sideof the superstrate) that is designed to advantageously reflect/absorbparticular colors of the solar spectrum to create an estheticallyappealing custom color to the visible surface of the solar panel (solarpanel art or architectural appeal).

The final metal contact and laser cell scribing can be formed in situ inone or more auxiliary chambers (or secondary chambers). The one or moreauxiliary chambers may be in-line with the primary MBE vacuum chamber.The primary MBE vacuum chamber may be the primary semiconductordeposition chamber. The metal contact and concomitant cell scribing maybe formed in situ by transferring the photovoltaic device of FIG. 5 fromthe primary MBE vacuum chamber to the one or more auxiliary in-linechambers under vacuum. The metal contact layer can have a thicknessbetween about 10,000 Å and 20,000 Å.

In embodiments, the tandem junction solar cell device comprises a thinZnTe or MgTe buffer layer over the superstrate (with or without theTCO). The tandem junction solar cell may also comprise a thin, highdoped p-type ZnTe (p+ ZnTe) layer over the buffer layer. The highlyresistive buffer layer can have a thickness less than or equal to about300 Å, or less than or equal to about 200 Å, or less than or equal toabout 100 Å. The high doped p-type ZnTe layer can have a thickness lessthan or equal to about 300 Å, or less than or equal to about 200 Å, orless than or equal to about 100 Å. The two layers can each be depositedover the superstrate at a deposition temperature between about 200° C.and about 400° C., or between about 250° C. and about 350° C. In apreferable embodiment, the two layers are formed via molecular beamepitaxy (“MBE”) at a growth rate of about 1 Å per second.

In a preferable embodiment, the high doped ZnTe layer is doped in situwith nitrogen to produce a p+ material layer having a nitrogen dopingconcentration between about 1×10¹⁹ cm⁻³ and about 1×10²⁰ cm⁻³.

Following formation of the buffer and high doped layers, an optionalcrystallizing anneal may be performed at a temperature differencebetween about 25° C. and 125° C., or between about 50° C. and 100° C.above the higher layers' (superstrate) deposition temperature. Thecrystallizing anneal can be performed for a time period less than orequal to about 5 minutes, or less than or equal to about 3 minutes, orless than or equal to about 1 minute. During the anneal, all depositionsources are closed. Following the anneal, the superstrate is returned tothe deposition temperature and the deposition is continued.

After forming the buffer and high doped layers, a Cd_(x)Zn_(1-x)Telight-absorbing layer (also “absorber layer” herein) may be grown as ap-type and n-type homojunction. P-type doping can be achieved with theaid of arsenic; n-type doping can be achieved with the aid of indium.The p-type Cd_(x)Zn_(1-x)Te light absorbing layer can have a thicknessof between about 0.25 microns and about 0.75 microns. The p-typeCd_(x)Zn_(1-x)Te light absorbing layer can be formed at a depositiontemperature between about 200° C. and about 400° C., or between about250° C. and about 350° C. In a preferable embodiment, theCd_(x)Zn_(1-x)Te layer is doped in situ with arsenic to produce a p-typematerial layer having an arsenic activated doping concentration betweenabout 1×10¹⁷ cm⁻³ and about 4×10¹⁷ cm⁻³. The n-type Cd_(x)Zn_(1-x)Telayer can have a thickness between about 0.25 microns and about 0.5microns. The n-type Cd_(x)Zn_(1-x)Te light absorbing layer can be formedat a deposition temperature between about 200° C. and about 400° C., orbetween about 250° C. and about 350° C.

In a preferable embodiment, the Cd_(x)Zn_(1-x)Te layer is doped in situwith indium to produce an n-type material layer having an indiumactivated doping concentration between about 1×10¹⁸ cm⁻³ and about4×10¹⁸ cm⁻³. In a preferable embodiment, the n-type Cd_(x)Zn_(1-x)Telayer is formed immediately following formation of the p-typeCd_(x)Zn_(1-x)Te layer. For instance, while forming the p-typeCd_(x)Zn_(1-x)Te layer by exposing the solar cell structure to a Cdsource, a Te source, a Zn source, and an As source, the As source can beshut off (or turned off) and an In source can be immediately introduced.

Following formation of the p-type CdZnTe light-absorbing layer (p-nCdZnTe homojunction), an optional crystallizing and grain boundarypassivating anneal can be performed at a temperature difference betweenabout 25° C. and 125° C., or between about 50° C. and 100° C. above theCdZnTe deposition temperature. The crystallizing and grain boundarypassivating anneal can be performed for a time period less than or equalto about 10 minutes, or less than or equal to about 5 minutes, or lessthan or equal to about 3 minutes. In a preferable embodiment, thecrystallizing and grain boundary passivating anneal is performed under aCd and As overpressure. All other sources of material flux are shut off(or closed off) during this anneal. In another embodiment, thecrystallizing and grain boundary passivating anneal is performed under aCd and Zn overpressure with an optional As overpressure. All othersources of material flux are closed off during this anneal. In anembodiment, the crystallizing and grain boundary passivating anneal isperformed more than once and at predetermined intervals during formationof the CdZnTe light-absorbing layer. In such a case, the crystallizingand grain boundary passivating anneal can be performed at CdZnTelight-absorbing layer thickness steps between about 0.2 microns andabout 0.8 micron or between about 0.4 microns and about 0.6 microns, fora time period less than or equal to about 3 minutes, or less than orequal to about 2 minutes, or less than or equal to about 1 minute peranneal, and followed by a return to the deposition temperature andcontinuation of the deposition of the Cd_(x)Zn_(1-x)Te light absorbinglayer. In an embodiment, the crystallizing and grain boundarypassivating anneal is performed during formation of the p-type CdZnTelayer but not during formation of the n-type CdZnTe layer. In yetanother embodiment, the crystallizing and grain boundary passivatinganneal is performed during formation of both the p-type and n-typeCdZnTe layers with As overpressure replaced with In overpressure for then-type CdZnTe layer anneal.

In a preferred embodiment, the Cd:Ze ratio is adjusted to provide acut-off wavelength between 610 nm and 650 nm (2.0 eV band-gap energy and1.9 eV band-gap energy, respectively) for the Cd_(x)Zn_(1-x)Te p-nhomojunction.

Next, a high doped n-type CdZnTe (n+ CdZnTe) layer may be formed overthe p-n CdZnTe layer. In the illustrated embodiment of FIG. 5, the highdoped CdZnTe layer is doped in situ with indium to produce an n+material layer.

Next, a high doped p-type CdTe (p+ CdTe) layer is provided over the n+CdZnTe layer and below the p-n CdTe layer. In the illustrated embodimentof FIG. 5, the high doped CdTe layer is doped in situ with arsenic toproduce a p+ material layer. The n+ CdZnTe layer and the p+ CdTe layerdefine a tunneling heterojunction for providing electrical contactbetween the p-n CdZnTe layer and the p-n CdTe layer.

The n+ CdZnTe of the tunneling junction can have the same Cd, Zn and Tecomposition (and be formed at the same deposition temperature) as then-type CdZnTe layer of the p-n CdZnTe layer below. The n+ CdZnTe layercan have a thickness less than or equal to about 100 Å; it can be dopedn-type with the aid of Indium at a concentration greater than or equalto about 5×10¹⁹ cm⁻³. The p+ CdTe layer of the tunneling junction canhave a thickness less than or equal to about 50 Å; it can be dopedp-type with the aid of Arsenic at a concentration greater than or equalto about 3×10¹⁸ cm⁻³.

The CdTe light absorbing p-n junction can be formed at a depositiontemperature between about 200° C. and about 400° C., or between about250° C. and about 350° C. In a preferable embodiment, the p-type CdTelayer is doped in situ with arsenic to produce a p-type material layerhaving an arsenic activated doping concentration about 1×10¹⁷ cm⁻³ to4×10¹⁷ cm⁻³. The p-type CdTe layer can have a thickness between about0.75 microns and about 1.0 microns. In a preferable embodiment, then-type CdTe layer is doped in situ with indium to produce an n-typematerial layer having an indium activated doping concentration about1×10¹⁸ cm⁻³ to 4×10¹⁸ cm⁻³. The n-type CdTe layer can have a thicknessbetween about 0.25 microns and about 0.50 microns. In a preferableembodiment, the n-type CdTe layer is formed immediately followingformation of the p-type CdTe layer. For instance, while forming thep-type CdTe layer by exposing the solar cell structure to a Cd source, aTe source and an As source, the As source can be closed off and an Insource can be immediately introduced.

Following formation of the p-type CdTe light-absorbing layer, anoptional crystallizing and grain boundary passivating anneal can beperformed at a temperature difference between about 25° C. and 125° C.,or between about 50° C. and 100° C. above the p-type CdTe depositiontemperature. The crystallizing and grain boundary passivating anneal canbe performed for a time period less than or equal to about 7 minutes, orless than or equal to about 5 minutes, or less than or equal to about 3minutes. In a preferable embodiment, the crystallizing and grainboundary passivating anneal is performed under a Cd and As overpressure.All other sources of material flux are closed off during this anneal. Inan embodiment, the crystallizing and grain boundary passivating annealis performed more than once and at predetermined intervals duringformation of the p-type CdTe light-absorbing layer. In such a case, thecrystallizing and grain boundary passivating anneal can be performed atCdTe light-absorbing layer thickness steps between about 0.2 microns andabout 0.8 micron or between about 0.4 microns and about 0.6 microns, fora time period less than or equal to about 3 minutes, or less than orequal to about 2 minutes, or less than or equal to about 1 minute, peranneal, and followed by a return to the deposition temperature andcontinuation of the deposition of the CdTe light absorbing layer. In apreferred embodiment, the n-type CdTe light-absorbing layer does notrequire an anneal.

Next, a high doped n-type CdTe (n+ CdTe) layer may be formed on the p-nCdTe layer. In the illustrated embodiment of FIG. 5, the n+ CdTe layeris a CdTe:In⁺⁺ layer. The n+ CdTe layer can have a thickness less thanor equal to about 300 Å, or less than or equal to about 200 Å, or lessthan or equal to about 100 Å. It can be formed at a temperature betweenabout 200° C. and about 400° C., or between about 250° C. and about 350°C. In an embodiment, the n+ CdTe layer has an Indium concentrationbetween about 1×10¹⁹ cm⁻³ and 1×10²⁰ cm⁻³. The n+ CdTe layer can provideohmic contact between the p-n CdTe layer and an overlying metal contact.

An optional metal contact layer can provide the final contact betweenthe CdTe layers (light absorbing CdTe layers and the n+ CdTe layer) andthe metallization of the backside of the structure. The final metalcontact layer may be formed by exposure of the PV cell to Sb and Tesources of flux or Hg and Te sources of flux, with all other sources ofmaterial flux closed off. In an embodiment, the SbTe or HgTe layer canhave a thickness less than or equal to about 300 Å, or less than orequal to about 200 Å, or less than or equal to about 100 Å. The SbTelayer can be deposited at a deposition temperature between about 200° C.and about 400° C., or between about 250° C. and about 350° C. In apreferable embodiment, the deposition temperature of the SbTe layer isthe same as the deposition temperature of the CdTe layers. The HgTelayer can be deposited at a deposition temperature between about 150° C.and about 250° C.

The final metal contact and laser cell scribing can be formed in situ inone or more auxiliary chambers (or secondary chambers). The auxiliarychambers may be in-line with the primary MBE vacuum chamber. The primaryMBE vacuum chamber may be the primary semiconductor deposition chamber.The metal contact and concomitant cell scribing may be formed in situ bytransferring the photovoltaic device of FIG. 5 from the primary MBEvacuum chamber to the auxiliary in-line chambers under vacuum. The metalcontact layer can have a thickness between about 10,000 Å and 20,000 Å.

The structure of FIG. 5 includes dual p-n junctions capable of absorbingsolar light at wavelengths from near ultraviolet (“UV”) to about 850 nm,and creating electricity by the flow of charge generated when the p-njunctions are exposed to light. Various embodiments of the inventionprovide in situ methods for forming low ohmic metal contacts to thefront and backside of the dual p-n junctions solar cell, high doping ofthe absorber layers, passivation of the grain boundaries,compositionally-graded hetero structures (as necessary), and highaccuracy control of layer thicknesses and junction location, in order tooptimize the extraction of photon-generated current and open circuitvoltage when the absorber layer of the dual p-n junctions solar cell isexposed to light.

The dual (“tandem”) p-n junctions structure of FIGS. 5 and 6 can beformed in a vacuum chamber configured for molecular beam epitaxy(“MBE”). The MBE chamber may be attached to one or more other vacuumchambers for forming one or more layers of the p-n junction structure.For instance, the MBE chamber may be attached to a vacuum chamberconfigured for forming the metal contact via sputtering or e-beamevaporation and a vacuum chamber configured for performing the lasercell scribing. In an alternative embodiment, multiple in-line vacuumchambers can be arranged to provide particular layer depositions of theoverall device structure, with increases in overall through-put.

With reference to FIG. 6, in an alternative embodiment of the invention,the p and n doping configuration of the tandem junction solar celldevice of FIG. 5 is reversed, providing n-p material layers. In theillustrated embodiment of FIG. 6, a tandem junction solar cell devicecomprises, from bottom to top, a superstrate, an optional TCO layer overthe superstrate, and an optional ZnTe or MgTe buffer layer. A high dopedn-type CdZnTe layer (n+ CdZnTe doped with In, or CdZnTe:In⁺⁺, asillustrated) is formed over the buffer layer or directly onto the TCO orbare glass substrate.

With continued reference to FIG. 6, the tandem junction solar celldevice further comprises a CdZnTe homojunction (n-p CdZnTe layer) formedover the high doped n-type CdZnTe layer. The CdZnTe homojunctioncomprises an n-type CdZnTe layer and a p-type CdZnTe layer formed overthe n+ CdZnTe layer. The tandem junction solar cell device furthercomprises a CdZnTe/CdTe tunneling heterojunction formed over the CdZnTehomojunction, the CdZnTe/CdTe tunneling heterojunction comprising a highdoped p-type CdZnTe (p+ CdZnTe) layer and a high doped n-type CdTe (n+CdTe) layer. The p+ CdZnTe layer can be doped with As (CdZnTe:As⁺⁺, asillustrated) and the n+ CdTe layer can be doped with In (CdTe:In⁺⁺, asillustrated).

The tandem junction solar cell device of FIG. 6 further comprises a CdTehomojunction (n-p CdTe layer) formed over the CdZnTe/CdTe tunnelingheterojunction. The CdTe homojunction comprises an n-type CdTe layerformed over the n+ CdTe layer of the tunneling heterojunction. The CdTehomojunction further comprises a p-type CdTe layer formed over then-type CdTe layer of the homojunction.

A high doped p-type CdTe (p+ CdTe) layer is provided over the p-typeCdTe layer of the CdTe homojunction. In the illustrated embodiment ofFIG. 6, the p+ CdTe layer is doped with As. An optional metal contactlayer can provide the final contact between the CdTe layers (lightabsorbing CdTe homojunction and the p+ CdTe layer) and the metallizationof the backside of the tandem junction solar cell structure. The finalmetal contact layer may comprise Sb and Te (i.e., SbTe or Sb₂Te₃) or Hgand Te (i.e., HgTe). The SbTe or HgTe layer may be formed via themethods discussed above in the context of FIG. 5. The SbTe or HgTe layercan have a thickness less than or equal to about 300 Å, or less than orequal to about 200 Å, or less than or equal to about 100 Å. In anembodiment, the SbTe layer is formed of Sb₂Te₃. A final metal contactcan be provided over the p+ CdTe layer (and optional SbTe or HgTelayer). The metal contact can have a thickness between about 10,000 Åand 20,000 Å.

The thicknesses and compositions of one or more layers of the tandemjunction solar cell structure of FIG. 6 may be similar to thosediscussed above in the context of the tandem junction solar cellstructure of FIG. 5. As an example, the p-type CdZnTe layer of theCdZnTe homojunction of FIG. 6 may have the same thickness as the n-typeCdZnTe layer of the CdZnTe homojunction of FIG. 5. In addition, one ormore layers of the tandem junction solar cell structure of FIG. 6 may beformed via methods discussed above in the context of FIG. 5.

Formation of one or more layers of the dual p-n junction structures ofFIGS. 5 and 6 may be achieved via any MBE technique known in the art orsimilar high vacuum techniques that provide a free-streaming flux ofelements or reactive molecules. The flux of elements may be adjusted toprovide a deposition rate less than or equal to about 20 microns/hour,or less than or equal to about 10 microns/hours, less than or equal toabout 1 micron/hour, depending on the layer being deposited. In apreferable embodiment, the flux of elements may be adjusted to provide adeposition rate between about 6 microns/hour and 10 microns/hour for thebulk p-n junctions and back contact layer growths and a deposition rateless than or equal to about 1 micron/hour for the initial thin bufferlayer and high doped p-type or n-type starting layer. MBE may be used toproduce a polycrystalline material structure with a total thicknessbetween about 1.5 micrometers (“microns”) and about 4 microns on anoptically transparent superstrate, e.g., a transparent conductiveoxide-coated glass superstrate, at a deposition temperature betweenabout 200° C. and about 400° C., or between about 250° C. and about 350°C., on a superstrate area greater than or equal to about 0.36 m² (i.e.,a superstrate having a length and/or a width greater than or equal toabout 600 mm). In an embodiment, the layers are grown at the sametemperature or within 25° C. of each other. In an embodiment, the totalstructure thickness is about 1.5 microns. In an embodiment, thesuperstrate area is greater than or equal to about 1 m².

The metal contact layer(s) for any of the cell structures describedherein can be formed of any elemental metal (e.g., Cu, Ni, Ti, Mo, Pt)with a relatively large work function, or a plurality of metal layers.In one embodiment, one or more of the metal contact layers discussedabove may be formed by depositing Ti, Ni and Au layers in sequence, witheach layer having a thickness between about 3000 and 6000 Angstroms.

Any of the structures and/or methods described herein may be combined.For example, a tandem solar cell structure may be provided with similarlayers to n-p junction solar cells or reverse p-n junction solar cells.As another example, a tandem solar cell may be provided where n-typelayers are deposited earlier (e.g., so they are closer to thesuperstrate layer) than the corresponding p-type layers. Similarly, anyof the structures described herein may have additional optional layers.For example, tandem cells may be provided with additional layers ofp-type and n-type paired layers, and/or tunneling layers. Or in someinstances, tandem solar cells or reverse p-n junction solar cells mayinclude an intrinsic layer, comparable to those described in variousembodiments. In another example, a tandem solar cell may have any numberof p-n layers, n-p layers, n-intrinsic-p layers, or p-intrinsic-nlayers. As another example, a solar cell may have a p-n layer, n-player, n-intrinsic-p layer, or p-intrinsic-n layer. Any of thestructures formed by the steps described herein, at any stage of theformation, may constitute an intermediate step or structure to a solarcell.

While in various embodiments reference has been made to a superstrate,any suitable substrate material may be used. In some embodiments, thevarious superstrate layers in FIGS. 1-6 can be substrate layers. Inpreferable embodiments, the substrate is configured to permit light topass through and to the various layers of the PV device. In analternative embodiment, the substrate is sufficiently opaque, and the PVdevice is configured to receive light from the opposite, non-substrateend of the PV device.

It should be understood from the foregoing that, while particularimplementations have been illustrated and described, variousmodifications can be made thereto and are contemplated herein. It isalso not intended that the invention be limited by the specific examplesprovided within the specification. While the invention has beendescribed with reference to the aforementioned specification, thedescriptions and illustrations of the preferable embodiments herein arenot meant to be construed in a limiting sense. Furthermore, it shall beunderstood that all aspects of the invention are not limited to thespecific depictions, configurations or relative proportions set forthherein which depend upon a variety of conditions and variables. Variousmodifications in form and detail of the embodiments of the inventionwill be apparent to a person skilled in the art. It is thereforecontemplated that the invention shall also cover any such modifications,variations and equivalents.

What is claimed is:
 1. A method for forming a photovoltaic device,comprising: forming a first layer over a substrate, the first layercomprising tellurium (Te) and zinc (Zn) and not containing cadmium (Cd);forming a second layer over the first layer, the second layer comprisingCd and Te; and forming a third layer over the second layer, the thirdlayer comprising Cd, Zn and Te.
 2. The method of claim 1, furthercomprising forming a layer comprising Zn and Te over the substratebefore forming the first layer.
 3. The method of claim 1, furthercomprising forming an other layer over the third layer, the other layercomprising Cd, Zn and Te.
 4. The method of claim 1, wherein the firstlayer is chemically doped n-type, the second layer is chemically dopedn-type, and the third layer is chemically doped p-type.
 5. The method ofclaim 1, wherein the first layer is chemically doped p-type, the secondlayer is chemically doped p-type, and the third layer is chemicallydoped n-type.
 6. A method for forming a photovoltaic device, comprising:forming a low ohmic ZnTe:N++ contact layer over a superstrate; forming ap-type CdTe layer over the low ohmic ZnTe:N++ layer; forming an n-type,compositionally graded, CdZnTe layer over the p-type CdTe to form a p-nheterojunction; and forming a low ohmic CdZnTe:In++ contact layer overthe p-n heterojunction.
 7. The method of claim 6, further comprisingforming a ZnTe buffer layer over the superstrate before forming theZnTe:N++ contact layer.
 8. A method for forming a photovoltaic device,comprising: forming a low ohmic ZnTe:N++ contact layer over asuperstrate; forming an intrinsic CdTe (i-CdTe) layer over the low ohmicZnTe:N++ layer; and forming a low ohmic CdZnTe:In++ contact layer overthe i-CdTe layer.
 9. The method of claim 8, further comprising forming aZnTe buffer layer over the superstrate before forming the ZnTe:N++contact layer.
 10. A method for forming a high performance singlejunction photovoltaic device, comprising: forming a low ohmic CdTe:In++contact layer over a superstrate; forming an n-type CdTe layer over thelow ohmic CdTe:In++ layer; forming a p-type, compositionally graded,CdZnTe layer over the n-type CdTe to form a p-n heterojunction; andforming a low ohmic CdZnTe:As++ contact layer over the p-nheterojunction.
 11. The method of claim 10, further comprising forming aZnTe buffer layer over the superstrate before forming the CdTe:In++contact layer.
 12. A method for forming a photovoltaic device,comprising forming an n-type layer having Cd and Te; forming anintrinsic CdTe layer over the n-type layer; and forming a p-type layerhaving Te and one or more of Cd and Zn over the intrinsic CdTe layer.13. The method of claim 12, wherein the photovoltaic device comprises asuperstrate below the n-type layer.
 14. The method of claim 12, whereinthe photovoltaic device comprises a superstrate above the p-type layer.15. A method for forming a photovoltaic device, comprising: forming afirst layer over a substrate that does not contain CdTe, the first layercomprising cadmium (Cd) and tellurium (Te), wherein the substrate isamorphous; forming a second layer over the first layer, the second layercomprising Cd, Zn and Te; and forming a third layer over the secondlayer, the third layer comprising Te.
 16. The method of claim 15,wherein the first layer is chemically doped n-type, the second layer ischemically doped p-type, and the third layer is chemically doped p-type.17. The method of claim 15, wherein the third layer does not contain Cd.18. A method for forming a photovoltaic device, comprising: forming afirst layer over a substrate, the first layer comprising tellurium (Te)and cadmium (Cd), wherein the first layer does not contain zinc (Zn),wherein the substrate is amorphous; forming a second layer over thefirst layer, the second layer comprising Cd, Zn and Te; and forming athird layer over the second layer, the third layer comprising Te. 19.The method of claim 18, wherein the substrate does not contain CdTe. 20.The method of claim 18, wherein the first layer is chemically dopedn-type, the second layer is chemically doped p-type, and the third layeris chemically doped p-type.
 21. The method of claim 18, wherein thethird layer does not contain Cd.